'root' is a link to the input file, not the cache file name

The `'root'` element of the cached object is the filename of the input file not the filename of the cache.

https://github.com/leafo/lessphp/blob/master/lessc.inc.php#L1939

Using the comparison `=== $cacheFile` causes the less to be re-compiled on every page load because it will never match.